Benzodiazepine (BZ) receptors are classified into central and peripheral benzodiazepine receptors. A peripheral benzodiazepine receptor (PBR) was at first confirmed in the periphery but its presence in the central nervous system was noted as well. It has been further clarified that PBR has a high density in the central nervous system and the density is same as or even higher than that of a central benzodiazepine receptor (CBR) in the same region. According to recent studies, it has been reported that PBR is present in microglia cells in the brain and increases in psychoneural diseases such as Alzheimer's disease where microglia is activated in the brain.
In a 11C-labeled substance of N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)-isoquinoline-3-carboxamide (hereinafter, referred to as PK11195) which is a conventional PBR ligand, its usefulness in diagnosis of glioma in the brain and Alzheimer's disease has been reported. However, its accumulation to the brain is very low and there is a problem for a quantitative analysis. In an image processing of distribution of PBR in a living human brain using a positron emission tomograph (PET), there is a demand for the development of a PBR ligand being able to obtain a high signal and, since N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (hereinafter, referred to as DAA1106) (JP-A-11-171844, the term “JP-A” as used herein means an “unexamined published Japanese patent application”) has a strong affinity and a high selectivity, it has been known to be suitable for such an object.